(1) Field of the Invention
The present invention relates to a rotorcraft rotor, and to a rotorcraft having such a rotor.
The invention thus lies in the technical field of rotorcraft rotors. More specifically, the invention lies in the field of abutments fitted to such rotors in order to limit the flapping movement of lift elements carried by the rotor.
(2) Description of Related Art
A rotorcraft usually includes at least one rotor for providing it with at least part of its lift and possibly also with its propulsion.
A rotor has a hub that is set into rotation by a mast. The hub then carries at least two lift assemblies. Consequently, each lift assembly comprises a blade connected to the hub via at least one retention and mobility member. Each blade may in particular comprise a lift element fastened to a cuff or a lift element having an incorporated cuff.
By way of example, a retention and mobility member includes a hinge referred to as a “spherical abutment”. Each spherical abutment may have one strength member fastened to the hub, and another strength member fastened to a cuff, regardless of whether the cuff is or is not incorporated in a lift element.
The blades are thus substantially plane elongate elements, each carried at one end by the hub, and extending transversely to the axis of rotation of the hub. The blades are mounted on the hub by individual mounting members for mounting the blades on the hub, which members include retention and mobility members for connecting the lift assemblies to the hub.
The hub may be a hub having a plurality of plates comprising two plates that are secured to a mast. For example, each retention and mobility member may have one strength member fastened to the plates by means of a pin.
In another embodiment, the hub may have a single plate secured to the mast, with the plate being provided with radial arms including openings. A retention and mobility member is optionally positioned in each opening, being secured to the plate by one of its strength members. A cuff then extends on either side of the plate going from the retention and mobility member to a lift element.
The mounting members are fitted respectively to each of the blades at their roots in order to enable the blades to be maneuvered by an operator. Mounting the blades movably on the hub makes it possible for a rotorcraft pilot to cause the pitch of the blades to vary collectively or cyclically in flight in order to influence the behavior of the rotorcraft with respect to its lift and/or its propulsion.
The freedom of the blades to move relative to the hub allows them to perform lead/lag movements, pitch variation movements, and up and down flapping movements. The concepts of up and down should be considered relative to the direction in which the axis of rotation of the rotor extends. When the rotor is rotating at nominal speed, the blades are spontaneously driven upwards under the effects of centrifugal force and of lift. When the rotor is stopped, the blades are subjected to no force other than gravity, and the blade therefore naturally droop downwards under the effect of their weight.
The rotor is thus commonly fitted with an abutment mechanism for limiting the extent to which the blades can move, in particular in flapping, under the effect of forces external to the rotorcraft.
In one embodiment, the abutment mechanism may comprise, for each blade, a bottom abutment member and a top abutment member that form obstacles to the individual flapping movements of the blades respectively downwards and upwards. The abutment mechanism includes bottom and top abutment tracks arranged on a retention and mobility member. For example, one strength member of a spherical abutment carries an abutment pad presenting an abutment track.
In the event of excessive flapping movement of a blade, an abutment track comes into contact with an abutment member, thereby limiting the flapping movement of the blade.
A particular function of the abutment mechanism is to limit the flapping movement of the blades when starting the rotor, and more particularly in the presence of wind. The abutment mechanism may also limit the flapping movement of the blades during an operation of folding the blades. In particular, when spherical abutments are present for hinging a blade to the hub, the abutment mechanism acts on the ground to tend to preserve the spherical abutments by ensuring that flapping movements of blades in the absence of centrifugal force do not lead to delamination of the elements constituting the spherical abutments.
While the rotor is rotating, the top abutment members should not interfere with upward mobility of the blades. The abutment mechanism may then include means for retracting the top abutment members when the rotor is in operation.
The top abutment members are then mounted on the hub to move between two positions. A first position is a position in which the top abutment members engage corresponding abutment tracks, so that when the rotor is stopped, they limit the flapping mobility of the blades between the bottom abutment members and the corresponding top abutment members. A second position is a position in which the top abutment members are disengaged, so that in flight the blades are free to move upwards in flapping.
To summarize, a top abutment mechanism usually includes one abutment member per lift assembly, one abutment track carried by a lift assembly, e.g. by its retention and mobility member, and one retraction means capable of positioning an abutment member in register with an abutment track, when necessary.
Furthermore, the abutment mechanism includes a low abutment ring common to all of the blades. This low abutment ring is movable in a groove.
In flight, only one blade at a time can move downwards, thereby pushing the ring in an opposite direction. The ring therefore does not prevent the lift elements from flapping.
In contrast, on the ground, when stopped or when rotating at low speed, the lift assemblies come simultaneously into contact with the ring, thereby causing them to be mutually prevented from moving.
An abutment mechanism of a blade thus includes a low abutment ring common to all of the lift elements, and, for each lift element, a respective high abutment that is retractable.
Those two abutment systems are independent and lead to a large number of mechanical parts in a space that, by its very nature, is restricted.
Furthermore, the low abutment ring may present weight that is not negligible on a heavy aircraft, and it may generate unbalance that needs to be taken into consideration.
Document FR 2 584 997 describes an abutment system having a central abutment that is common to all of the lift elements. That central element cannot be eclipsed in flight.
Furthermore, the system includes an abutment member arranged on each retention and mobility member. The abutment member rises in elevation over a vertical height that is greater than the height of the central abutment. Under such circumstances, the abutment member includes a top nib and a bottom nib representing flapping limits for the corresponding lift element.
Distantly-related technologies do not make use of abutments, but rather of hooks.
Thus, Document U.S. Pat. No. 2,719,593 describes an aircraft having a plurality of blades hinged to a rotor head in particular via a “drag” hinge. A blade also has a “flapping” link.
For each blade, the aircraft has a flapping retention mechanism for holding the blade in a given position. That mechanism includes a hook connected by vertical arms to the flapping link, being arranged under the head of the rotor. The hook is provided with a weight so that under the effect of centrifugal force it tilts about a horizontal axis of rotation that is hinged to the vertical arms.
The hook is then engaged about a pin of the drag hinge when the rotor is stopped, and it disengages from that orifice when the rotor is set into rotation.
Document FR 2 551 723 describes a blade fastened to a retention and hinge member, the retention and hinge member being connected to two plates of a rotor by a fastener pin.
That document describes a mechanism having an abutment comprising a perforated plate through which said fastener pin passes. The abutment is also secured to a weight for causing it to turn about said pin under the effect of centrifugal forces. That abutment co-operates with a bottom bearing surface of a pitch control yoke of a lift assembly.
Furthermore, the mechanism includes a hook suitable for being engaged in the bottom bearing surface.
Under such circumstances, the mechanism has a pivot abutment and a hook suitable for catching a bottom bearing surface of a pitch control yoke of a lift assembly. That mechanism then tends to replace a high abutment with a hook co-operating with a pitch control yoke.
Document FR 2 447 856 describes a rotor having a plurality of blades.
Each blade has a cuff. The cuff comprises a U-shaped yoke that is hinged to a hub having a plate by means of a retention and hinge member.
Furthermore, an actuator lever is provided for modifying the pitch angle of the blade. Two appendices of the lever are then secured to the yoke.
Each blade is provided with a mechanism for limiting its flapping movement, where appropriate. The mechanism includes a bracket rigidly fastened to the plate, the bracket having a top projection and a bottom projection that acts as abutments.
In addition, the mechanism has a cylindrical element that is movable in a direction that is substantially perpendicular to the axis of rotation of the rotor and parallel to the longitudinal axis of the blade. The cylindrical element is received in an orifice of an appendix of the actuator lever. Each abutment is thus stationary and co-operates with a movable member of an actuator lever.
Also known are Documents FR 2 735 094, GB 2 073 121, and EP 1 371 554.